1. Field of the Invention
The present invention relates to a control system for an automatic transmission in which occurrence of shock due to a select operation during running of a vehicle is suppressed.
2. Description of the Prior Art
A gear change of the automatic transmission is executed by actuating, of a plurality of frictional engaging elements such as clutches, brakes and the like, an engaging element corresponding to a desired gear position. Operation or non-operation of the engaging elements is performed by supply or discharge of hydraulic pressure to and from the engaging elements, and supply speed or discharge speed of the hydraulic pressure is adjusted by a solenoid so as to control engagement or disengagement of the frictional engaging elements and reduce occurrence of shock.
As such an example, in Japanese Patent Application No. 2001-21653, the applicant has proposed to control a pressure regulating valve by means of a solenoid valve, in which a current signal to be inputted to the solenoid has a stepwise waveform, so that a working pressure to be supplied to an engaging element can be set to a desired value, to modify a rapid rise or fall of waveform of an hydraulic pressure command value and absorb vibrations by filtering the current signal according to each shifting stage, so as to suppress a delay or surge occurring in an actual hydraulic pressure.
However, though the above technique has merits described above, the occurrence of shock cannot be prevented for the following reason, when a driver switches a range selector from a D range to an N range and returns it back to the D range again during the running of the vehicle.
In short, even if the driver switches the range selector from the D range to the N range during the running, the above current signal is unchanged and the pressure regulating valve is held at a maximum working pressure supply position. However, a manual valve is switched because of the select operation to the N range, and the working pressure which has been supplied to the engaging elements till then is discharged from the manual valve.
If the D range is selected again in this condition, a command for lowering current signal to be inputted to the solenoid is issued in order to reduce the working pressure for the purpose of reducing select shock, as in the above select operation from the N range to the D range. However, since this current signal of which reduction was commanded is also filtered, an actual current to be applied to the solenoid generates a delay from the time when the reduction of the current signal is commanded and reduces gradually.
As a result, while a spool of the pressure regulating valve slightly returns from the maximum working pressure supply position due to this delay, an original pressure is outputted at once from the manual valve switched by the select operation and is supplied to the engaging elements with maintaining a high pressure without being reduced to a desired degree by the pressure regulating valve, as a result of which engagement shock occurs.
This condition is shown in FIG. 7. At a time t0 when the range selector is switched from the N range to the D range, a hydraulic pressure command value Pio corresponding to the current signal is rapidly lowered and then gradually increased at once, as indicated by the thick solid line. However, since the current signal is filtered with a strong filter, an actual hydraulic pressure command value Pi corresponding to the actual current is gradually lowered till a time td more slowly than the hydraulic pressure command value Pio, as indicated by the broken line. Therefore, within a time period t0xcx9cTd, the rise of a working pressure P exceeds the hydraulic pressure command value Pio, as indicated by the thin solid line, and acts on the engaging elements, thereby causing the occurrence of shock.
It is therefore an object of the present invention to provide a control system capable of suppressing shock which occurs when a driver switches a range selector from a D range to a N range and returns it back to the D range during the running of a vehicle.
To accomplish the above object, the present invention provides a control system for an automatic transmission, comprising: a manual valve capable of outputting an original pressure depending on a select operation of a driver; a pressure regulating valve capable of supplying a working pressure to be obtained by regulating the original pressure which is supplied from the manual valve to an engaging element to be engaged in a D range; a solenoid for supplying a variable control pressure to the pressure regulating valve and controlling a magnitude of the working pressure to be outputted by the pressure regulating valve; and a control unit for supplying a filtered current to the solenoid, said control unit selecting a filtering process using a weak filter when the driver switches a range selector from the D range to an N range and then again returns it to the D range during running of a vehicle and returning said filtering process using a weak filter to a filtering process using a strong filter after a predetermined time elapsed.
Since a current is applied to the solenoid after the filtering process is changed to the filtering process using a weak filter at the time of the select operation from the N range to the D range, the pressure regulating valve is rapidly switched in a direction in which the working pressure reduces, and a high hydraulic pressure from the manual valve does not act on the engaging elements, thereby preventing a select shock at the time of the select operation during the running of the vehicle.
The above and further objects and features of the invention will be more fully apparent from the following description when the same is considered with reference to the accompanying drawings.